This invention relates to a process for the production of light fast, transparent casting elastomers using novel catalyst combinations. The polyurethanes produced according to the present invention may be used for various purposes, e.g. as coating or sealing compounds, in packaging foils or in the bio-medical field.
It is known that polyurethane systems based on aromatic diisocyanates (e.g. tolylene diisocyanate, diisocyanatodiphenyl methane, etc.) undergo severe yellowing under prolonged exposure to light, even if they have been light stabilized, and therefore do not fulfill the requirements for the areas of usage mentioned above. It is also known that polyurethanes which are based on aliphatic or alicyclic polyisocyanates, in particular polyurethane lacquers of this type, generally do not show such a tendency to yellowing.
Using aliphatic or alicyclic polyisocyanates for the production of light fast transparent polyurethane elastomers has hitherto failed to be used commercially because of the severe cloudiness which occurs in the course of the polyaddition reaction, the inadequate heat resistance and the inferior mechanical properties of the elastomers.
In German Auslegeschrift No. 1,745,418 there is described a process for the production of cellular or non-cellular molded polyurethane articles based on aromatic or aliphatic polyisocyanates in which the catalysts used for the polyisocyanate polyaddition reaction are diazabicyclo compounds, e.g. 1-8-diazabicyclo-(5,4,0)-undecene or its ammonium salts, optionally in combination with organo-metallic compounds, such as dibutyl tin dilaurate.
The use of such catalysts or catalyst combinations or of other catalysts, such as diazabicyclo-(2,2,2)-octane, for the production of molded polyurethane articles based on aliphatic or alicyclic polyisocyanates does not improve the heat resistance or mechanical properties of the elastomers and also results in cloudy products permeated with bubbles. The formation of bubbles may only be prevented by using very high catalyst concentrations (with a drastic reduction in the pot-life of the polyurethane mixtures which makes normal processing impossible). However, the use of high catalyst concentrations cause discoloration, as well as catalyzing the back reaction.
It was therefore an object of the present invention to provide a new process by which it would be possible to produce light fast, transparent elastomers based on aliphatic or alicyclic polyurethanes which would not have the disadvantages of cloudiness, formation of bubbles and low heat resistance and which could be cured in the heat or cold and processed under suitable conditions (sufficiently long pot-life and short molding time).
It has now surprisingly been found that the disadvantages mentioned above could be obviated and elastomers having excellent mechanical properties could be obtained in an operating cycle suitable for practical purposes if particular catalyst combinations were used.